I live in a mobile home part time in Palm Springs CA (I go there every other weekend). I hardly ever turn on the air conditioning or heating system when I am there, but wondered how often they need to be serviced to make sure they are in good working order when I do use them.

You didn't mention if it was oil, gas or electric.

These are absolutes.

Your job is to keep the furnace filter changed and that interval is variable. Electronic media filter, once a year. Some others, one a month.

The second part is to keep the outside unit coils clean and get the leaves out of it. That may require professional help. In many cases yearly.

Inspect the cooling drainage systems at least once a year.

This can put off professional cleaning of the coil outside and rust.
And it can virtually eliminate the need to have the inside coil cleaned.

Whole house duct cleaning is another maintenance item, but that may be almost never.

Thank you very much for taking the time. I need to take care of this now!!